Higher plants are exposed to various types of environmental stress, such as high temperature, low temperature, dryness, salt, and metal. Among various types of environmental stress, heat stress due to high temperature imposes a serious damage on plant growth, and it accordingly significantly affects agricultural production in the world (Peet, M. M., Willits, D. H., 1998, Agric. Forest Meteorol., 92, pp. 191-202; Hall, A. E., 2001, Crop Responses to Environment, CRC Press LLC, Boca Raton, Fla.). According to the report made by the Intergovernmental Panel on Climatic Change, the world mean temperature increases by 0.3° C. every century due to global warming. It is predicted that the mean temperature becomes 3° C. higher than the current level on 2100 and such problem becomes more serious in the future (Jones, P. D., et al., 1999, Rev. Geophys., 37, pp. 173-199; Porter, J. R., 2005, Nature 436, p. 174).
As year-round cultivation of tomatoes becomes popular, various barriers to the growth, such as fruit set failure caused by lowered pollen fertility, decreased size, and deteriorated quality of tomatoes due to high temperature in summer, have been reported. Pollen development failure and lowered pollen fertility are major causes of fruit set failure in high temperature season. In order to overcome such problems, heat tolerance has been imparted to plants via various techniques. As a chemical spraying-based technique, a method for reducing high-temperature stress under high-temperature conditions and promoting healthy growth of plants by spraying an agent for imparting high-temperature-stress tolerance for agricultural/horticultural use to plants has been known (Japanese patent publication No. H11-199419 A (1999)). As a genetic engineering-based technique, a method for imparting heat tolerance to transgenic tobacco plants by causing the overexpression of tomato-derived mitochondrial small heat-shock proteins has been known (Japanese patent publication No. 2002-95370 A). As ingenuity in facilities and methods for cultivation, in addition, a developed apparatus for effectively cooling only buds under development stage including stamens that are most sensitive to high temperature to an optimal temperature for plant growth and a method for promoting fruit set and growth of plants using it are known (International Publication No. WO 2007/058347). However, such conventional techniques for imparting heat tolerance to plants are problematic in terms of economic efficiency, environmental load, and/or workability. Accordingly, breeding a heat-tolerant variety that exhibits stable fruit set even in high temperature season is urgently required. In addition, a variety capable of efficiently developing a normal fruit with seeds even under high temperature conditions is required as a parent for breeding a heat-tolerant variety. However, no techniques for efficiently breeding such heat-tolerant variety have yet been established.